The present invention is in the field of crosslinkable polyethylenes; more particularly the present invention relates to a crosslinkable polyethylene composition and a method of rotational molding using the crosslinkable polyethylene composition.
Rotational molding, also known as rotomolding, is used in the manufacture of hollow objects from thermoplastics. In the basic process of rotational molding, polymer is placed in a mold. The mold is first heated and then cooled while being rotated. The mold can be rotated uniaxially or biaxially and is usually rotated about two perpendicular axes simultaneously. The mold is heated externally and then cooled while being rotated. Included in polymers useful in rotational molding are a variety of polyolefins such as polyethylene including crosslinkable polyethylene. A general discussion of rotational molding is given in "Modern Plastics Encyclopedia 1979-1980", Vol. 56, No. 10A, beginning at page 381. Rotational molding has a feature such that it can result in hollow articles which are, as compared with those obtained by blow molding methods, complicated, large in size and uniform in wall thickness, and further the material loss is minor.
Compositions of interest relating to crosslinking polyethylene are disclosed in U.S. Pat. Nos. 3,372,139; 3,806,555; 3,876,613; 3,974,132; 4,018,852; 4,028,332; and 4,267,080. Crosslinkable polyethylene rotational molding compositions are disclosed in U.S. Pat. Nos. 3,876,613 and 4,267,080.
Polyethylene is commonly polymerized using two types of catalyst systems. The first is a chromium based system which results in a polymer having terminal unsaturation. The second type of catalyst is a Ziegler-type catalyst which results in the polymer having terminal groups which have substantially complete terminal saturation. Additionally, polymers formed with the Ziegler-type catalysts have been found to contain residue acidic compounds, most commonly chlorides. It has been found that crosslinkable polyethylene formed with use of the Ziegler-type catalysts cannot be used in critical molding operations, such as rotational molding. Articles rotationally molded from crosslinkable polyethylene made with a Ziegler-type catalyst exhibit satisfactory surface appearance, mold release, and impact properties when molded using methods disclosed in the art such as in U.S. Pat. Nos. 3,876,613 and 4,267,080.